1. Field of the Invention
An electric toaster oven is conventionally used to prepare a number of different food items by the application of heat. Since different food items may respond in different ways to the application of heat from an upper and a lower heat source, adjustments must sometimes be made to assure that all portions of the food item are properly heated in the toaster oven. For example, while most portions of a cake may be baking at the proper rate for a particular heat setting, the crust may become scorched unless special precautions are taken. Conversely, for a properly browned crust, the remainder of the cake may be receiving insufficient heat to bake properly throughout.
In many existing toaster ovens the shelf which supports the food item while it is exposed to heat, usually a wire grate or grill or the like, is adjustable to vary the distance between the food item and the upper and lower heat sources. Although this arrangement affords some flexibility, it is not useful where a change must be made during the cooking process. Thus, it is rarely convenient, while the food item is baking or broiling etc., to open the oven and to physically lower or raise the hot shelf while the food item is supported on it, since this requires withdrawing the shelf from the oven and reinserting it on a higher or lower set of supports. However, a change in the manner in which heat is applied is sometimes required during the cooking process, e.g. during the final phase of baking a cake when the rate at which heat is applied may have to be reduced in order to obtain a cake crust of the desired color.
In essence, the repositioning of the food item relative to the upper and lower heat sources changes the heating ratio, i.e. the rate at which heat is applied by the upper heat source relative to that applied by the lower heat source, while the rate of total heat applied by both sources remains constant. The total heat applied can, of course, be changed by the temperature setting, which is a conventional feature of most existing toaster ovens.
2. Description of the Prior Art
It is already known in the art to vary the heating ratio between the upper and lower sources of an oven by electrical means in order to meet the requirements of the particular food item being baked, e.g. as shown by U.S. Pat. No. 1,685,647 to Shroyer. Further, as shown both by Shroyer and U.S. Pat. No. 4,302,661 to Perry, each heat source may consist of multiple heating elements, the interconnection of which may itself be varied to obtain different effects, e.g. broiling, baking or self-cleaning in Perry, or to provide slow, intermediate or high heat in Shroyer. However, existing prior art equipment fails to provide such an arrangement wherein the rate of total heat applied to the food item remains constant. As a consequence, other factors will vary as the interconnection of the heating elements is changed, e.g. the degree to which the food item is cooked in a given time interval, or the length of the time interval required for the food item to reach the desire condition.
A known technique by which the ratio of heat applied by the upper heat source relative to the lower heat source is varied in existing toaster ovens provides a diode rectifier which is selectively connected in series with the upper heating elements when the heat applied by the latter is to be reduced. A disadvantage of the technique is that a relatively large net DC current is drawn, on the order of 4 amperes or higher, which can cause saturation in the secondary winding of the power transformer. Where this occurs, an unbalanced power source results.
Another known technique for varying the heating ratio involves replacing the upper and lower heating elements, or any one of them, with heating elements having a different electrical resistance. Since this requires disconnecting and reconnecting one or more heating elements, the technique is considered impractical for the average person, for being too difficult to carry out.
Still a further technique involves the use of a rheostat or of a variable transformer connected to the upper heating elements to vary the magnitude of the current in the latter. When so connected, some of the power normally dissipated in the heating elements is then dissipated in the rheostat, which must be relatively large to handle the current. This mandates an increase in the size as well as in the cost of the toaster oven and is therefore undesirable.